The present invention relates to electromagnetically operated valve units, and more particularly to an improvement of an electromagnetically operated valve unit which includes a solenoid coil wound around a tubular yoke of magnetic material, a plunger of magnetic material axially movable within the tubular yoke and facing at its outer circumference the inner circumference of the yoke with a predetermined annular clearance to provide a magnetic circuit across the yoke and plunger, a valve assembly cooperating with the plunger to control the quantity of fluid flow passing therethrough, and resilient means for biasing the plunger in one direction to normally close the valve assembly.
In such a conventional valve unit as described above, when the solenoid coil is energized by an electric current from an electric power source, a linear attraction force is generated in proportion to the value of the electric current between the inner circumference of the yoke and the outer circumference of the plunger. Thus, the plunger is displaced by the linear attraction force against the biasing force of the resilient means to control the opening degree of the valve assembly. In operation of the valve unit, the length of the moving-stroke of the plunger is limited by the magnetic saturation characteristic of the plunger. For this reason, when the amount of magnetic flux across the yoke and plunger reaches the value of magnetic saturation due to increase of the outer circumference area of the plunger facing the inner circumference of the yoke upon by displacement of the plunger, the linear attraction force does not increase, and the displacement of the plunger is restricted by the biasing force of the resilient means before arrival of the plunger at the end of its maximum stroke. On the other hand, it is required to keep the axial length of the valve unit as short as possible in a simple construction and to permit smooth displacement of the plunger so as to ensure a good performance of the valve unit.